The present invention relates to a method and circuit for driving a brushless motor, and a method and circuit for detecting the rotational position of a brushless motor.
Brushless motors are highly efficient motors with a simple structure that have recently attracted attention. For example, a claw pole brushless motor includes a laminate of U-phase, W-phase, and W-phase annular stator members, and U-phase, V-phase, and W-phase coils wound between the stator members. The coils are connected by delta connection or by Y-connection to form the three-phase claw pole brushless motor.
Each of the U-phase, V-phase, and W-phase stator members includes a plurality of claw poles arranged to face a rotor of the motor. The claw poles of the different phases are arranged in a predetermined order in the circumferential direction. A three-phase inverter circuit controls equilibrium currents for three phases that are supplied through the three-phase coils. With the controlled equilibrium currents flowing through the coils, the claw poles of the different phases each generate a rotating magnetic field, which drives and rotates the rotor.
In recent years, brushless motors that have a simpler structure, a smaller size, and cost less have been proposed (refer to, for example, Japanese Laid-Open Patent Publication No. 2007-215305). Such a brushless motor may eliminate one of the U-phase coil, V-phase coil, and W-phase coil and include only two phases of coils. The simplified motor structure eliminating one phase can improve productivity and reduce the size and cost of the motor.
A brushless motor eliminating one phase and including only two phases of coils may use four power elements and two direct current power supplies to control the currents supplied through the two phases of coils and rotate the rotor (refer to, for example, WO 2007/010934).
However, when the equilibrium currents for three phases are supplied from the three-phase inverter circuit to the terminals of the two phases of coils in the brushless motor described in Japanese Laid-Open Patent Publication No. 2007-215305, the different phases of claw poles will generate a rotating magnetic field that is not circular but elliptical in which the magnitude of the field changes depending on the electrical angle.
In this manner, the brushless motor eliminating one phase and including only two phases of coils fails to generate a circular rotating magnetic filed. As a result, the brushless motor cannot avoid a secondary torque ripple generated in accordance with the electrical angle of the motor and thus cannot avoid noise and vibration generated by such a secondary torque ripple.
Moreover, the brushless motor described in WO 2007/010934 requires two direct current power supplies to achieve the simplified structure including only two of the three phases of coils. The need for the two DC power supplies complicates the circuit structure of the motor and makes the motor unpractical.